A semiconductor device such as an LED or a power module has a structure in which semiconductor elements are bonded onto a circuit layer made of a conductive material.
In a power semiconductor element for high power control used to control wind power generation, an electric automobile, a hybrid automobile, and the like, a large amount of heat is generated. Thus, as a substrate for mounting the power semiconductor element thereon, for example, a power module substrate including a ceramic substrate made of aluminum nitride (AlN), alumina (Al2O3), and the like and a circuit layer formed by bonding a metal plate having excellent conductivity to one surface of the ceramic substrate has been conventionally widely used. As the power module substrate, a power module substrate in which a metal layer is formed by bonding a metal plate to the other surface of the ceramic substrate has also been provided.
For example, PTL 1 proposes a power module substrate obtained by bonding an aluminum plate, which becomes a circuit layer, to one surface of a ceramic substrate made of aluminum nitride (AlN) via an Al—Si-based brazing filler material and bonding an aluminum plate, which becomes a metal layer, to the other surface of the ceramic substrate via an Al—Si-based brazing filler material.
Such a power module substrate is configured such that a semiconductor element as a power element is mounted on the circuit layer via a solder layer and is used as a power module. In addition, a heat sink made of copper may be bonded to the metal layer side via solder.
When the above-described power module is used, there is the burden of a heat cycle. At this time, stress caused by the difference in the thermal expansion coefficient between the ceramic substrate and the aluminum plate is applied to the bonding interfaces between the ceramic substrate and the circuit layer and the metal layer, and thus there is a concern of the bonding reliability deteriorating. Conventionally, a circuit layer and a metal layer are composed of aluminum having a purity of 99.99 mass % or higher (so-called 4N aluminum) or the like and thermal stress is absorbed via the deformation of the circuit layer and the metal layer so that bonding reliability is improved.
In the case in which the circuit layer and the metal layer are composed of aluminum having a purity of 99.99 mass % or higher (4N aluminum) or the like and having a relatively weak deformation resistance, when loading a thermal cycle, there arises a problem of waviness or wrinkles occurring on the surfaces of the circuit layer and the metal layer. When waviness or wrinkles occur on the surfaces of the circuit layer and the metal layer as described above, cracks may be formed in the solder layer, and thus the reliability of the power module may deteriorate.
Particularly, in recent years, from the viewpoint of an environmental load, Sn—Ag-based and Sn—Cu-based lead-free solder materials have been frequently used for a solder layer. Since these lead-free solder materials have strong deformation resistance compared to conventional Sn—Pb-based solder materials, cracks are likely to be formed in the solder layer due to waviness or wrinkles occurring on the circuit layer and the metal layer.
In addition, in recent years, since the application environments of power modules have become harsher and the amount of heat generated from electronic components such as a semiconductor element has become greater, temperature differences in a heat cycle have become greater, and waviness or wrinkles are likely to occur on the surfaces of the circuit layer and the metal layer.
For example, PTL 2 proposes a power module substrate in which waviness or wrinkles on the surface of the circuit layer are prevented by forming a circuit layer using a precipitation dispersion type aluminum alloy.
Further, PTL 3 proposes a power module substrate in which plastic deformation of a metal layer is prevented by adding additive elements to aluminum constituting the metal layer.